SAN FRANCISCO — The US National Security Agency (NSA) chief on Thursday urged top computer security specialists to harden the nation’s critical infrastructure against inevitable destructive cyber attacks.

“This is an important time,” NSA and Cyber Command director Gen. Keith Alexander said during a presentation at a premier RSA Conference in San Francisco.

“Most of the destructive tools being developed haven’t been used; we need to use this window of opportunity to develop defenses.”

Two days earlier, Deputy Secretary of Defense William Lynn gave a similar warning, saying the capability clearly exists for malicious software to cause real-world damage at power plants, water supplies and other vital points.